1. Field of the Invention
The present invention relates to a wavelength tunable semiconductor laser and fabrication process thereof. More specifically, the invention relates to a wavelength tunable semiconductor laser which is important for application to optical communication, optical measurement.
2. Description of Related Art
Importance for the wavelength tunable semiconductor laser has been growing in application to wavelength division multiplexing transmission system, wavelength division switching system, wavelength cross-connection system and so forth of trunk system of an optical communication, as well as in the field of optical measurement.
Bitrate of optical fiber communication has been increased significantly, in the recent years. Currently, the transmission bitrate has reached to be restricted by the bandwidth of electric circuit at transmitter and receiver portions. Therefore, as means for easily increasing transmission capacity without being electrically restricted with respect to the transmission speed, wavelength division multiplexing (WDM) transmission system has been attracting attentions. Also, wavelength division optical switching system, wavelength cross connection system and so forth have currently been attracting attentions. In these system, a wavelength tunable semiconductor laser plays an important role. It is desirable that the wavelength tunable laser has a function to permit continuous wavelength control by a single control current or control voltage. If two or more control currents are present, complicate control becomes necessary for obtaining the desired wavelength.
One of such wavelength tunable laser is a distributed Bragg reflector (DBR) semiconductor laser. The prior art DBR semiconductor laser will be briefly discussed with reference to FIG. 1. It should be noted that the prior art similar to that illustrated in FIG. 1 have been disclosed in Japanese Unexamined Patent Publication (Kokai) No. Showa 61-54690 or S. Murata et al. "Electronics Letters", Vol. 23, No. 8, pp 403 to 405, published on Apr. 9, 1987, for example. As seen from FIG. 1, 3 regions, i.e. an active region, a phase control region and DBR region are divided in a direction of resonator. In the active region, an active layer 10 is provided. On the other hand, in the phase control region and the DBR region, a tuning layer 104 is provided. Also, in the DBR region, a deflection grading 110 is provided in the vicinity of the tuning layer 104. On respective region, electrodes 111, 112 and 113 are formed for permitting independent injection of current to respective regions. In FIG. 1, I.sub.a denotes a laser current, and I.sub.t denotes a tuning current. In case of such 3 section DBR laser, the wavelength is shifted toward short wavelength with accompanied lasing mode jumps by increasing current applied to the DBR region, as shown by the wavelength tuning characteristics of FIG. 2. During jumping of the mode, the wavelength range can be covered by adjusting the current applied to the phase control region.
However, in view point of application of the wavelength tunable semiconductor laser to the actual system, controlling of two currents (phase control current and DBR current) in order to obtain laser oscillation at desired wavelength is required a complicated operation. Therefore, it is desirable to make it possible to set the desired wavelength by a single control current. Continuous wavelength control in the DBR laser, as disclosed in Japanese Unexamined Patent Publication No. Showa 61-54690 as set forth above, can be obtained by satisfying the following relationship. EQU .DELTA.n.sub.d /.DELTA.n.sub.p =Lp/(Lp+La) (1)
wherein .DELTA.n.sub.d and .DELTA.n.sub.p are variation amounts of equivalent refraction indexes due to constant injection at the DBR region and the phase control region, Lp and La are lengths of the phase control region and the active region. By optimizing the ratio of the injection currents to the phase control region and the DBR region by a resistance division for satisfying the foregoing equation (1), continuous wavelength tuning operation becomes possible. By this method, maximum 3.8 nm of continuous wavelength control operation has been reported in O. Ishida et al. "Electronics Letters" Vol. 30, No. 3, pp 241 to 242, published on Feb. 3, 1994.
However, in such current division method, it is difficult to maintain the ratio between the phase control current and the DBR current, constant. Therefore, it is possible to cause jump of the mode at the midway of wavelength tuning operation as shown in FIG. 3, which show wavelength variation versus tuning current for current divion method. In order to avoid this, it is effective to set the resistance value for current division sufficiently greater than the series resistance of the phase control and the DBR region. However, there is such case, another problems, such as difficulty in establishing impedance matching required for high speed switching of the lasing wavelength.